1. Field of the Invention
The present invention relates to a vision measuring device which measures a measuring target by imaging the measuring target.
2. Description of the Related Art
A vision measuring device having an auto-focusing function sequentially acquires images of a measuring target by moving an imaging device such as a camera or its optical system in the light axis direction, and defines the position in the light axis direction at which an image with the highest contrast is acquired as the in-focus position for the measuring target (JP2009-168607A).
The auto-focusing accuracy and speed of such a vision measuring device having a contrast-type auto-focusing function are more or less influenced by the frame rate, etc. in the auto-focusing operation.
For example, such a vision measuring device might acquire images with uneven brightness due to flickers which might be caused depending on the combination of the frame rate of the imaging device and the emission frequency of an illumination unit that illuminates the imaging range. In this case, the contrast of an image with poor brightness becomes low regardless of whether the image is acquired at an in-focus position or not because such an image has a small brightness difference between the pixels, whereas the contrast of an image with affluent brightness becomes high. Hence, there is a problem that it is difficult to judge the in-focus position based on the contrast, which reduces the reliability of auto-focusing.
In order to overcome this problem, a vision measuring device commonly has an illumination unit that does not cause flickers depending on its combination with the imaging device. However, also in this case, there may be many occasions when it is difficult to eliminate the influence of external diffused light depending on the configuration of the vision measuring device. A fluorescent tube (hereinafter, referred to as interior lamp) is an example of external diffused light that is difficult to eliminate and tends to be problematic. An interior lamp repeats blinking at 100 Hz in a district where the power source frequency is 50 Hz and at 120 Hz in a district where the power source frequency is 60 Hz, and gives an adverse effect to a measurement result and an auto-focusing result. There is also a case when an illumination unit itself flickers.
Moreover, although such a vision measuring device can be easily realized by a camera and software, there is a problem that the device takes a long time for the auto-focusing process. In order to solve this problem, it is conceivable to move the camera and the optical system at a high speed. However, this makes the image acquiring pitch during auto-focusing rough because the frame rate is constant, and makes it difficult to obtain a correct in-focus position.